About Me

Thursday, 7 January 2016

Radio-Isotope Dating: Age of the Earth and Plate Tectonics - Introduction

'The world was created on Sunday,
23rd October 4004 BC" …James Ussher, Irish Archbishop of Armagh'..., this world has neither a beginning nor an
end". …James Hutton (1726 – 1797)

For the past few years, I have been giving an
annual seminar to the Glasgow University Nuclear Physics Group on a topic that
has something to do with nuclear physics but definitely it would have no
overlap with the current research programme of the group. I am happy to say that this year I chose to
talk about radioisotope dating as applied to the determination of the age of
the Earth and its contribution in establishing plate tectonics on a firm
scientific basis. With no real
background in Geology, it was hard work but rather exciting to get to know
better the planet we live on.

In fact, the impact of radioisotope dating on geology
and the way we now see the historic evolution of the Earth can only be
described as dramatic. In 1963, geophysicist
John Tuzo-Wilson summarized: "It
will be difficult for most of us to accept that large amounts of what we have
written and taught has been erroneous".

To an extent, it is not surprising that geology
was in such a mess until about 1950.
There was really no absolute time scale - it was a qualitative subject -
not really a proper science. This has a feeling of deja vu. Physics suffered similar fate for almost 2000
years, biology and medicine are definitely showing signs of new life and are developing
real understanding about the basis of diseases and how to cure them.

Geology is the science that deals with the
history of the Earth as recorded in the rocks.
The problem has been that there was no clock, and rocks have the
habit of moving about, weathering and generally changing in every way possible
- typically over geological time scales - over millions of years. Until radioisotope dating arrived properly, around
1950, only thing we could do was to visually examine rocks and chemically find out
the minerals it contained. A totally
unsatisfactory situation which lends to a lot of speculative wacky hypotheses; and the Church got involved too!

Radioactivity was discovered in 1896 and the
first ideas that it can be used to date minerals in rocks came within a
decade. The first results were that many
of the rocks were hundreds of millions of years old - imagine the reaction of established
geologists who believed that the Earth was no more than 20 million years
old. The Church wanted us to believe
that it was a mere 6000 years old. The literature
is still full of young-earth 'scientists' who are spending lot of energy in
trying to prove that radioisotope dating is a flawed science and its results
are misleading. These are things that
make life unique.

It took 50 years to sort out all the details in
radioisotope dating method and it is now possible to say with good certainty
that the Earth was created at the same time when the Solar System formed from a
nebula 4.67 billion years ago. The
initial Earth was in a molten form and the crust took a few hundred million
years to form. Strictly, the Earth is
slightly younger than the Solar System. Meteorites,
remnants of the early structures formed in the Solar Nebula, have proved invaluable to
fix the date when the Solar System formed.

While we are talking about the formation of the
Solar System, it might be good to reflect on the age of the Milky Way - our
galaxy. It is considered that stars and galaxies
started to form within about 100 to 200 million years after the big bang that
created the Universe 13.7 billion years ago.
Independent determination of Be-9 abundance on the oldest stars in
globular clusters at the periphery of the Milky Way puts their age at 13.6 +-
0.8 billion years. Therefore, the Solar System is much younger than the Milky
Way.

Radioisotope
dating provided an absolute clock and it was then possible to date past events
as they happened on the Earth. One of
the most remarkable discovery was the confirmation of sea floor spreading and
thus establishing the theory of continental drift or plate tectonics on a firm
scientific footing. Over the next few
weeks, I shall be discussing some of these subjects from a physicist view point.